let s = " / 2 3 4 ? / "
    // 替换
    print("空格替换成-:", s.replacingOccurrences(of: " ", with: "-"))
    // 过滤
    print("空格过滤掉:", s.replacingOccurrences(of: " ", with: ""))
    // 去首尾空格
    print("去掉空格:", s.trimmingCharacters(in: .whitespaces))
    // 分割
    print("分割:", s.components(separatedBy: "/"))
    // 拼接
    let a = ["1", "2", "3"]
    print("拼接:", a.joined(separator: "-"))

字符串截取：

let timeString = "2016.12.12"
        
let timeString = "2016.12.12"

let index = timeString.index(timeString.endIndex, offsetBy: -2)
let suffix = timeString.substring(from: index)

let index1 = timeString.index(timeString.startIndex, offsetBy: 2)
let prefix = timeString.substring(to: index1)

let startRange = timeString.index(timeString.startIndex, offsetBy: 5)
let endRange = timeString.index(timeString.startIndex, offsetBy: 7)
let middle = timeString.substring(with: Range(startRange..<endRange))

Range使用：
let str = “反正我是帅的不要不要！！！你呢？"

let startIndex = str.startIndex.advancedBy(3) //参数必须大于等于0 
//以前用法  let startIndex = advance(self.startIndex, r.startIndex)
let endIndex = str.endIndex.advancedBy(-3) //参数必须小于0

let range = Range<String.Index>(start: startIndex, end: endIndex)
swift3.0改变为：
let range = Range(startIndex..<endIndex)
print(str.startIndex)//起始位置
print(str.endIndex)//结束位置
print(str.substringWithRange(range))//截取字符串
print(str[startIndex])//开始位置的字符
print(str[endIndex])//结束位置的字符

//建议转成把String转成NSString....

        let string = sender.text!
        var count = 0
        for c in string.characters {
            print(c)
            print(c.debugDescription)
            if c.debugDescription.validLeterAndFigure {
                count += 1
            } else {
                count += 2
            }
        }

Range使用举例:

左到右截取
startIndex to offsetBy正数
右到左截取
endindex from offsetBy负数
func ga_dateToDay() -> String {
    
    let index = self.index(self.endIndex, offsetBy: -2)
    let suffix = self.substring(from: index)
    // substring(to: index)
    
    let startRange = self.index(self.startIndex, offsetBy: 5)
    let endRange = self.index(self.startIndex, offsetBy: 7)
    let middle = self.substring(with: Range(startRange..<endRange))
    // 月
    let middleInt = Int(middle)
    // 日
    let suffixInt = Int(suffix)
    
    let calendar = Calendar(identifier: .gregorian)
    let components = calendar.dateComponents([.month, .day], from: Date())
    // 月
    let month = components.month
    // 日
    let day = components.day
    
    if month == middleInt {
        if day == suffixInt {
            return "今天"
        }
        if day! > suffixInt! {
            if day! - suffixInt! == 1 {
                return "昨天"
            }
            return self
        }
    } else {
        if month! - middleInt! == 1 {
            if day == 1 {
                return "昨天"
            }
        }
    }
    
    return self
}